The challenge of locating a thermally stable polymer film with excellent barrier properties as well as good mechanical properties for use in a broad range of applications has led researchers in varied directions. Both monolithic and multi-component, multi-layer films have been constructed; however, to date, no suitable materials have been available which provide the unique combination of thermal stability, strength, thinness and, most importantly, barrier properties as demonstrated by resistance to water vapor permeation.
One attempt to solve this problem is taught in U.S. Pat. No. 6,465,103 B1, to Tsai et al., which is directed to highly oriented multilayer films produced by coextruding or laminating at least one layer of PCTFE (polychlorotrifluoroethylene) fluoropolymer, at least one layer of a polyolefin homopolymer or copolymer and an intermediate adhesive layer of a polyolefin having at least one functional moiety of an unsaturated carboxylic acid or anhydride thereof. The polyolefin layer allows the fluoropolymer layer to be stretched up to ten times its length to orient the fluoropolymer film and increase mechanical properties and water vapor properties of the film. Commercially available films of this construction are sold under the trade name ACLAR® by Honeywell Corporation. However, limitations exist with respect to these materials, including the presence of the polyolefin and adhesive layers which contribute undesirable thickness to the final film and added cost during processing. Moreover, these films have limited chemical and temperature resistance (e.g., maximum thermal stability reported for ACLAR® films is about 215° C.) and limited water vapor permeation resistance.
Other materials have also been evaluated for suitability in demanding barrier applications. For example, a polyvinylidene chloride (PVDC) copolymer film sold by the Dow Chemical Company (Midland, Mich.) under the trade name SARAN is widely known as a barrier film for protecting foods against oxygen, moisture and chemical attack, as well as other barrier applications. However, this PVDC film has limited chemical and temperature range (i.e., melt temperature of about 160° C.) and limited water vapor permeation resistance.
The advantage of using polytetrafluoroethylene (PTFE) in harsh chemical environments and over a broad range of temperatures is well known. PTFE has exhibited utility as a material for use in harsh chemical environments where other polymers quickly degrade. PTFE also has a useful temperature range from as high as 260° C. to as low as near −273° C. However, PTFE is characterized by poor mechanical properties such as low tensile strength, poor cold flow resistance or creep resistance, poor cut-through and abrasion resistance and a general poor mechanical integrity that precludes its consideration in many materials engineering applications.
Low porosity PTFE articles have been made in the past through use of a skiving process in which solid PTFE films are split or shaved from a thicker preformed article. These articles are characterized by low strength, poor cold flow resistance, and poor load bearing capabilities in both the length and width directions of the film. Processes including paste extrusion of PTFE fine powder have also been used to produce low porosity PTFE articles, however they are also characterized by relatively poor mechanical characteristics. Attempts have also been made to strengthen low porosity PTFE films by stretching in the length dimension. Strength gains are minimal and, by the nature of the process, are achieved in only a single dimension, thus greatly minimizing the utility of the film.
A PTFE material, specifically, expanded polytetrafluoroethylene, may be produced as taught in U.S. Pat. No. 3,953,566. This porous expanded polytetrafluoroethylene (ePTFE) has a microstructure consisting of nodes interconnected by fibrils. It is of higher strength than unexpanded PTFE and retains the chemical inertness and wide useful temperature range of unexpanded PTFE.
However, ePTFE is porous and hence cannot be used as a barrier layer to low surface tension fluids since such fluids with surface tensions less than 50 dyne-cm pass through the pores of the membrane. Compressed ePTFE articles are taught in U.S. Pat. No. 3,953,566 in which a platen press was used to densify a thin sheet of ePTFE with and without heat. However, cold flow occurred in the press, non-uniform parts resulted and a density of over 2.1 g/cc was not achieved. The ePTFE sheet used in U.S. Pat. No. 3,953,566 was stretched or strengthened in only one direction and, hence, the utility of the finished article was severely limited.
Similarly, U.S. Pat. No. 4,732,629, to Cooper et al., describes a method of increasing the cut-through resistance of a PTFE insulated conductor. Unsintered PTFE was expanded and compressed and then applied to a conductor. However, densities of 2.1 g/cc or greater were not achieved, and the resultant tensile strengths of the finished article were not reported for either the length or width directions.
U.S. Pat. No. 5,061,561 to Katayama describes a method to produce high density fibers from ePTFE; however, the method yielded an article that is significantly different from this invention and applicable only to fine filaments and not to sheets.
In U.S. Pat. No. 5,374,473 to Knox et al., a method is described for producing articles of densified ePTFE by placing two or more layers of porous ePTFE inside a heat and pressure stable flexible container, evacuating gas from the chamber, subjecting the chamber to a pressure of 150 to 350 psi and temperature from 368-400° C., then cooling the container while reducing pressure. The resulting densified structure is described as useful in such barrier applications as pump diaphragms when laminated to a flexible backer. While the Knox et al. materials exhibit improved barrier properties in the applications described, the methods and articles taught are limited to making thin, flexible PTFE films with uniformly good barrier properties (e.g., a water vapor permeation coefficient on the order of 0.10 g-mm/m2/day).
U.S. Pat. No. 5,792,525, to Fuhr et al., teaches forming creep resistant articles which are dimensioned from a stock material of one or more layers of expanded polytetrafluoroethylene which have been densified. The densified expanded PTFE material exhibits remnants of a fibril and node structure, and the resultant article is resistant to creep at high temperatures and under high loads. The stock material is preferably formed in the manner taught in U.S. Pat. No. 5,374,473, to Knox et al., described earlier herein. The shaped articles are then formed by any suitable method such as a heat forming process or a machine forming process. Compression molding and lathing are specifically described as shaping methods. Fuhr et al. does not teach or suggest the capability of forming thin PTFE films with good barrier properties.
WO 02/102572 A1 is directed to PTFE resin blow molded articles and resin blow molding methods. The PTFE starting material is drawn by blow molding at a temperature at or above the temperature at which PTFE begins to melt, which is a temperature where both crystalline and non-crystalline regions are present in the PTFE, to form a non-porous structure. From the teachings, this method and product are subject to significant variations in processing and product properties, and trial and error is necessary to determine the drawing temperature and draw ratio for each batch of material. In addition, significant limitations in material size and material strength would result based on the processing techniques taught.
Two products currently available from W.L. Gore and Associates, Inc. include a dense fluoropolymer film exhibiting barrier properties. The first product comprises a PTFE barrier layer bonded between two porous PTFE layers. The second product comprises a PTFE barrier layer bonded on one side to a thermoplastic layer such as FEP (fluoroethylene propylene), PFA (perfluoroacrylate) or THV (a polymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride). The barrier layer in these commercial products is a film of high water vapor resistance (i.e., low water vapor permeation) PTFE having good tensile properties in the orthogonal directions of width and length. It would be understood by an artisan of skill in the art that barrier performance and bulk density of a material are positively correlated. This barrier layer has a bulk density of 2.11 g/cc or greater, is substantially free of pores, has a matrix tensile strength of 10,000 psi or greater in both the width and length directions, and has a water vapor permeation coefficient of 0.018 g-mm/m2/day. While these materials have been successfully implemented in a number of applications requiring flexible, thin materials with good chemical resistance and water vapor permeation resistance, a need still exists for materials with further improved performance for even more demanding barrier applications.